Kernel: Add a beep() syscall that beeps the PC speaker.

Hook this up in Terminal so that the '\a' character generates a beep.
Finally emit an '\a' character in the shell line editing code when
backspacing at the start of the line.

Kernel/Scheduler.h

@@ -25,6 +25,7 @@ public:
     static void prepare_to_modify_tss(Thread&);
     static Process* colonel();
     static bool is_active();
+    static void beep();
 private:
     static void prepare_for_iret_to_new_process();
 };